How to find the open circuit voltage?

[ltnghia1304]

Homework Statement

How can we find the open circuit voltage of this one:

Thank you

Homework Equations

The Attempt at a Solution

 

[gneill]

By the forum rules, you need to show what you've tried before help can be offered. A genuine effort is required. Unless you show an attempt your post will be removed...

What equations are relevant for solving these types of problems in general?